1. Field of the Invention
The present invention relates to an image forming apparatus and a laser driving apparatus used in the image forming apparatus and, more particularly, to an image forming apparatus, suited to a copying machine, a laser beam printer, a facsimile, and the like, which guides an optically modulated laser beam from a laser beam source onto an image carrier such as an electrostatic recording medium so as to form an image, e.g., an electrostatic latent image, on the surface of the image carrier, and a laser driving apparatus used in the image forming apparatus. The present invention also relates to a copying machine, a laser beam printer, a facsimile, and the like using these apparatuses.
2. Related Background Art
A conventional image forming apparatus of this type uses a one-chip element such as a laser chip having a laser beam source, e.g., one laser diode, and a photodetector, e.g., one photodiode (PD) sensor. The image forming apparatus also has a pulse current source for driving the laser diode. In this apparatus, laser emission is stabilized as follows. Negative feedback for the pulse current source is done using a detection output signal from the PD sensor that has detected a laser beam, thereby controlling the pulse current amount and setting the laser beam amount to a predetermined value (this control will be referred to as APC hereinafter).
An image forming apparatus using a multilaser unit having a plurality of laser beam sources exploits this arrangement and uses an APC circuit having an arrangement similar to that shown in FIG. 1. FIG. 1 shows the internal arrangement of a semiconductor laser chip 201 in an optical system used for the image forming apparatus shown in FIG. 2. Referring to FIG. 1, the semiconductor laser chip 201 is a twin laser chip formed by an A laser (LD-A) 302, a B laser (LD-B) 303, and a PD sensor (PD) 304. A pulse current source 305 is used for the A laser 302, and light emission by the A laser 302 is ON/OFF-controlled by a switch 307 that is turned on/off in accordance with an image data signal DATA 1 output from an image processing section 316. Similarly, a pulse current source 306 is used for the B laser 303, and light emission by the B laser 303 is ON/OFF-controlled by a switch 308 that is turned on/off in accordance with an image data signal DATA 2 output from the image processing section 316.
The current output from the PD sensor 304 upon detection of a laser beam is converted into a voltage signal by a current/voltage converter (I/V) 309. This voltage signal is input to an operational amplifier 315 through a sample-and-hold (S/H) circuit 311 to be compared with a reference voltage Vrefl (313). The pulse current source 305 is controlled in accordance with an output voltage V.sub.H1 from the operational amplifier 315. The output signal from the PD sensor 304 is also input to an operational amplifier 314 through the I/V converter 309 and an S/H circuit 310 to be compared with a reference voltage Vref2 (312). The pulse current source 306 is controlled in accordance with an output voltage V.sub.H2 from the operational amplifier 314. The reference voltages 312 and 313 are target values for controlling the respective pulse current sources. The S/H circuits 311 and 310 are controlled in accordance with sample/hold signals S/H1 Sig and S/H2 Sig output from the image processing section 316.
Referring to FIG. 1, the image forming section 316 generates the aforementioned image data DATA 1 and DATA 2, and a system controller 317 controls the image forming section 316.
In the above multilaser unit comprised of a plurality of lasers, only one PD sensor is used. For this reason, to perform negative feedback control on the amount of pulse current for each laser, each pulse current source must be controlled after making the respective lasers emit light one by one time-divisionally.
FIG. 3 shows a conventional control sequence. First of all, the A laser 302 is driven to emit light at time T1 to control a pulse current amount I.sub.PA for the A laser 302. In this case, the A laser 302 is continuously turned on until its laser emission stabilizes. After the current amount I.sub.PA stabilizes, the pulse current is held, and the A laser 302 is temporarily turned off at time T2. The B laser 303 is then driven to emit light at time T3 to control a pulse current amount I.sub.PB for the B laser 303. As in the case of the A laser 302, the B laser 303 is continuously turned on until its laser emission stabilizes. After the current amount I.sub.PB stabilizes, the pulse current is held, and the B laser 303 is temporarily turned off at time T4. After that, the apparatus is ready to output image data.
At the start of this negative feedback control, the total time (between time t1 to time t4) required to obtain predetermined light amounts by making the lasers emit light is generally the control time per laser multiplied by the number of lasers incorporated in the multilaser unit. In an apparatus of this type, therefore, a certain preparatory time is required before image data is output. This may lead to a delay in image formation.